The present invention relates to the art of vehicle control systems, and more particularly to a method and apparatus for monitoring the operation of vehicle brakes on a vehicle equipped with an electronic braking system as is commonly used to provide traction control (TC) and/or an anti-lock braking system (ABS).
Modern vehicles commonly include traction control systems that enhance the directional stability of a vehicle and the total traction availability to the drive wheels in conditions where one or more of the drive wheels encounters a low coefficient of friction surface or otherwise develops a wheel spinning condition. Most motor vehicles incorporate a differential in which torque applied to the non-spinning wheel on one side of the vehicle is reduced when the drive wheel on the other side encounters a spinning condition. Therefore, total tractive effort is reduced.
With traction control, the speeds of the drive wheels are detected and compared with the other wheels on the vehicle. In the case of a vehicle that is also equipped with an anti-lock brake system, this comparison can be made with all of the wheels on the vehicle. To remedy a wheel spinning condition, the traction control system may reduce engine torque through a control link to the engine controller. In addition, such systems typically also activate the brake of the spinning wheel, gently pumping or modulating the brake to slow the spinning wheel speed. This brake application to the spinning wheel allows the drive differential to transmit higher torque to other drive wheels. Such traction control systems are found in passenger cars having hydraulic braking systems as well as heavy duty trucks employing pneumatic brake systems.
Since traction control systems require wheel speed inputs and active brake controllers, they are almost universally provided on vehicles also having ABS. In fact in many cases, traction control is provided as an enhancement to ABS with little or no additional hardware. Through appropriate control algorithms and software, an ABS controller can be operated to provide traction control.
Anti-lock brake systems are also generally well known. In these systems, a brake modulator valve is fluidically connected between a source of pressurized fluid (e.g., air) and the brake chamber or actuator. The modulator is typically a three-way valve that under normal service conditions receives pressurized fluid upon activation of the brakes (e.g., upon operator depression of a brake pedal) and conveys the pressurized fluid to the brake actuators. If an anti-lock event--i.e., impending wheel lock--is sensed by the associated an electronic control unit (ECU), the ECU sends electronic control signals to a solenoid valve assembly associated with the relevant modulator. The ECU controls the solenoid valve assembly in a particular manner so that the modulator valve associated with the solenoid valve assembly is caused to control the flow of pressurized fluid to the brake actuators in a manner that simulates brake pedal "pumping" at a repetition rate not obtainable by a human operator. The result is improved braking efficiency without loss of vehicle control associated with wheel lock or skid.
While anti-lock brake systems, traction control systems, and other electronic brake systems have enjoyed widespread commercial success, there has been found a need for a braking system that is also able to monitor the actual operation of the vehicle brake at the wheel end. For example, it is possible for the vehicle brake actuator or the associated brake pads or shoes or other brake components to become less effective over time due to wear or malfunction. Also, it is possible for a service technician to improperly service the brake components at a vehicle wheel end leading to decreased or inefficient application of braking force by the braking system at the affected wheel end. Prior braking systems include no means for detecting inoperable or deficient braking components at the vehicle wheel ends. With these prior systems, it is possible for a driver of such a vehicle to operate the vehicle indefinitely until the braking defect is noticed during a routine safety inspection or due to especially poor braking performance.
Accordingly, there has been deemed a need to develop a method and apparatus, fully compatible with ABS and traction control systems, that is able to monitor the operation of vehicle brake components at each wheel end and notify a vehicle operator and/or service technician upon a deficient brake assembly being detected.